The present invention generally relates to power quality regulation in electric power generation systems and, more particularly, to voltage control of a high reactance permanent magnet generator without a rotor position sensor in an electrical power generation system of the type used on aircraft.
An electrical power generation system representative of the type used on aircraft may generate electricity in the form of 3-phase power at an approximate frequency of 1000 Hertz (Hz), selected to optimize the weight and volume of the system, and an open circuit voltage of approximately 163 Volts alternating current (VAC), peak (pk). Power from such an aircraft electrical power generation system may be supplied, for example, from the alternating current (AC) power output of a generator providing 3-phase alternating current at 1000 Hertz and 163 VAC (pk), which may be passed through a solid state power converter, also commonly referred to as an inverter, and rectified, i.e. converted to direct current (DC), to provide a 270 Volt direct current (VDC) power source. The electrical power generation system may be used to power various subsystems and components, for example, electric motors, which can inject noise or power fluctuations into the electrical power generation system. For certain applications it is desirable to protect the generator from short circuit conditions, which may arise within the electric power generation system or the load connected to it. One approach for protecting the generator from short circuits is to design the generator with high reactance windings, i.e., windings that possess sufficient inductance such that the short-circuit current is limited to a value approximately equal to its rated value. A permanent magnet generator with high reactance windings is referred to as a high reactance, permanent magnet generator (HR-PMG).
The power quality of the DC voltage at the interface of the inverter with the electrical power generation system may be subject to certain requirements and constraints. For military aircraft, for example, the power quality of the DC voltage at the interface of the inverter with the electrical power generation system is typically specified by a military standard such as Mil-Std 704. Electrical generation systems on aircraft are also subject to requirements limiting the amount of electromagnetic radiation conducted emissions of the system, which may interfere with other electronics systems on the aircraft, and is referred to as electromagnetic interference (EMI). To meet EMI requirements, which are stringent for military aircraft in particular, electrical generation systems contain LC-type filters comprised of inductances and capacitances to filter out fluctuations, such as harmonics, in the current and voltage. For example, the electrical power generation system described above may require an EMI filter at the output of the inverter or may at least contain a capacitor bank at the output of the inverter. The LC filter circuits are prone, however, to harmonic resonance, i.e., such circuits may resonate at certain frequencies. For example, an electric motor powered by the electric power generation system may inject some amount of current harmonics into the generation system, despite interfacing with its feeder through appropriate EMI filters. The amplitude of the resonant currents circulating throughout the electrical power generation system may become so large as to create unacceptable voltage fluctuation, or ripple, at the output of the inverter. Such large voltage ripples are unacceptable because they interfere with voltage control of the electric power generation system, and may even interfere with voltage control to the extent of creating limit-cycle conditions, and because they exceed allowable power quality limits.
For these reasons, electric power generation systems generally include some means for regulating voltage and current levels to reduce power fluctuations to an acceptable level and maintain a substantially constant and dependable source of power. One means for regulating power is the use of a voltage controller, which may provide voltage commands to the inverter for adjusting its power output to compensate for the conditions causing the power fluctuations. The voltage commands are based on input from the DC portion of the power system and from the generator, for example, it may be necessary to know the position of the generator""s rotor. A rotor position sensor may be provided to supply an electrical signal indicating the generator rotor position to the voltage controller. The voltage controller can use the rotor position information along with other system information to provide gating signals to the inverter.
Rotor position sensors, which may be electro-magnetic devices, such as Hall effect sensors, or electro-optical devices, for example, are usually sensitive to, or intolerant of, the hostile operating environment provided by an HR-PMG or other type of generator. Many rotor position sensors, and in particular electro-optical sensors, are sensitive to dust, which is typically present in the generator environment. Electronic rotor position sensors, and other types of sensors, may be intolerant of the temperatures typically present in the generator environment. A high reactance, permanent magnet generator may operate at temperatures in the range of 150-180xc2x0 C., whereas electronic rotor position sensors are typically not tolerant of operating temperatures in excess of approximately 125xc2x0 C. Provision of rotor position sensors thus requires modification and compromise of the design of the generator, which can be expensive and still not provide rotor position sensing having satisfactory dependability. Thus, it is desirable to eliminate the rotor position sensor used by prior art voltage controllers, but the rotor position input is necessary for satisfactory voltage control of the HR-PMG.
As can be seen, there is a need for voltage control of a generator without a rotor position sensor in electrical power generation systems. There is also a need for voltage control of a high reactance permanent magnet generator without a rotor position sensor in electrical power generation systems of the type used on aircraft.
The present invention provides voltage control of a generator without a rotor position sensor in electrical power generation systems. In particular, the present invention provides voltage control of a high reactance permanent magnet generator without a rotor position sensor in electrical power generation systems of the type used on aircraft.
In one aspect of the present invention, an electrical power system includes an electric power source, for example, an electrical machine, capable of supplying AC power; a power converter connected between the power source and the distribution system; a rotor position estimator for the power source suitable for estimating the position of the rotor of the electrical power generator in a stationary reference frame; and a controller configured to provide commands to the gating logic of the inverter for controlling the power converter, the controller receiving a reference frame from the position estimator, DC link voltage from the output of the inverter, and power source phase current sensed information for the power source, from which the gating information for the inverter is computed.
In another aspect of the present invention, an electrical power system includes an electric power source comprising a high reactance generator adapted for providing AC power to a load; a power converter connected to the electric power source; two or more current sensors disposed for sensing an AC current between the electric power source and the power converter so that the current sensors can provide a current feedback signal; a position estimator for receiving the current feedback signal in Park vector format and providing a synchronous reference frame; a controller, which provides a voltage command for controlling the power converter, where the controller receives the current feedback signal and a current reference in Park vector format and uses the synchronous reference frame, the current feedback signal, and the current reference to produce the voltage command; and a modulation module, which receives the voltage command and provides control signals to the power converter.
The position estimator includes a multiplier which multiplies the current feedback signal in Park vector format by a rotator vector defined by unity amplitude and angle xcex8 in Park vector format; a PI-regulator which operates on the imaginary portion of this product to provide an estimated electrical frequency; and an integrator which integrates the estimated electrical frequency to provide the negative estimate of the transformation angle xcex8 to the multiplier and to provide the transformation angle xcex8, which is sufficient to determine the synchronous reference frame, to the controller.
In yet another aspect of the present invention, an electrical power system includes an electric power source comprising a high reactance permanent magnet generator adapted for providing AC power to a load at varying power factors; a power converter connected to the electric power source; two or more current sensors disposed for sensing an AC current between the electric power source and the power converter so that the current sensors can provide a current feedback signal; a position estimator for receiving the current feedback signal in Park vector format and providing a synchronous reference frame; a DC link voltage sensor for sensing a DC link voltage output of the power converter and providing a DC link voltage feed back signal; a first comparator receiving the DC link voltage feed back signal and a DC link voltage command signal, and producing a DC link voltage error signal; a PI-regulator for regulating the DC link voltage error signal to produce a DC link voltage angle component, where the DC link voltage angle component is a portion of the angle component of a current reference having an angle component and an amplitude component and whereby the electrical power system can control the DC link voltage output of the power converter; a current sensor disposed for sensing a DC current output of the power converter and providing a load current signal; a feedforward function module which receives the load current signal and produces a feedforward angle component; a combiner which combines the DC link voltage angle component and the feedforward angle component so that the angle component of the current reference includes the feedforward angle component so that gain sensitivity of the angle component of the current reference is minimized; a non-linear function generator configured to receive the load current signal and produce the amplitude component of the current reference, so that the controller can control the DC current output of the power converter; a controller, which provides a voltage command for controlling the power converter, where the controller receives the current feedback signal and a current reference in Park vector format and uses the synchronous reference frame, the current feedback signal, and the current reference to produce the voltage command; and a space vector modulation module, which receives the voltage command and provides control signals to the power converter.
The position estimator includes a multiplier which multiplies the current feedback signal in Park vector format by a negative estimate of a transformation angle xcex8 in Park vector format to provide a synchronous frame signal; a PI-regulator which regulates the imaginary portion of the synchronous frame signal to provide an estimated electrical frequency; and an integrator which integrates the estimated electrical frequency to provide the negative estimate of the transformation angle xcex8 to the multiplier and to provide the transformation angle xcex8, which is sufficient to determine the synchronous reference frame, to the controller.
The controller includes a first conversion block for providing the current feedback signal in Park vector format in the synchronous reference frame; a second conversion block for providing the current reference in Park vector format in the synchronous reference frame; a comparator which subtracts the current feedback signal from the current reference in the synchronous reference frame, to provide a signal operated upon by the PI-regulator producing the voltage command.
In a further aspect of the present invention, a method for electrical power generation includes the steps of: supplying electric power from an electric power source, which includes a high reactance generator, and in which the electric power source is connected to a power converter; sensing an AC current between the electric power source and the power converter using two or more current sensors to provide a current feedback signal; using the current feedback signal in Park vector format to provide a synchronous reference frame; and controlling the power converter by using the synchronous reference frame, the current feedback signal, and a current reference to produce a voltage command and feeding the voltage command through a modulation module to provide control signals to the power converter.
The step of using the current feedback signal in Park vector format to provide a synchronous reference frame includes performing steps of: receiving the current feedback signal in. Park vector format; multiplying the current feedback signal in Park vector format by a negative estimate of a transformation angle xcex8 in Park vector format to provide a synchronous frame signal; regulating the imaginary component of the synchronous frame signal with a PI-regulator to provide an estimated electrical frequency; and integrating the estimated electrical frequency to provide the negative estimate of the transformation angle xcex8 in the above step of multiplying and to provide the transformation angle xcex8, which provides the synchronous reference frame.